1. Field of the Invention
This invention is related to environmental and thermal barrier coating systems exposed to high temperatures such as the hostile thermal environment of a gas turbine engine. More particularly, the present invention is directed to a layer applied over the environmental layer or the thermal barrier layer of an airfoil that reduces the build-up of corrosive materials on the outer surface of the airfoil.
2. Discussion of the Prior Art
Many coating systems have been developed that improve the life of turbine airfoil components. Typically these components are made from high temperature superalloy materials and have an airfoil portion, a platform portion and a shank portion. These systems typically include an environmental coating applied over the airfoil portion of the superalloy substrate that extends into the hot gases of combustion to provide protection to the substrate from oxidation and corrosion typically experienced in this high temperature exhaust gas. In many cases, a ceramic thermal barrier coating is applied over the environmental coating to improve the high temperature response of the system. These coatings have been refined over time and new coatings have been developed for specific applications.
Early coatings for gas turbine applications included nickel aluminide coatings applied by a codeposition (Codep) process. As refinements to the coatings and processing have occurred, the preferred coating for protection against oxidation and hot corrosion that has evolved is a nickel-platinum aluminide application in which a layer of platinum is deposited over the substrate followed by an aluminizing application. The aluminide may be applied by a Codep process or by a vapor phase aluminide process. The resulting coating is a graded composition of (Ni,Pt)Al. For marine applications in which salt water environments present a problem due to the severe corrosion experienced, the MCrAlY class of coatings were developed, where M is an element selected from the group consisting of Ni, Co, Fe and combinations thereof.
As the oxidation and corrosion issues of the airfoil portion of the component were addressed by the coatings, corrosion problems in the platform portion and below the platform portion in the shank portion developed due to other improvements in turbine operation that has permitted an increase in the operating temperature of the engine and hence in the temperature of the exhaust gases. In order to address this problem, the (Ni,Pt)Al coating has been applied to the platform portion and the shank portion of the components. However, for a number of reasons, including but not limited to rubbing associated with rotating apparatus, the coating has not been fully effective in providing protection against corrosion in these regions. Despite improvements in coating compositions and processing technology, a build-up of corrosion as a result of interactions with hot combustion gases occurs in these regions. Corrosion as used herein includes oxidation as well as interactions of the substrate with other hot gases.
Various modifications to coating systems to improve coating performance have been proposed. These can include additional layers over the substrate. For example, U.S. Pat. No. 5,891,267 forms a carburized zone below an alumina layer in order to tie up refractory metals present in the underlying superalloy substrate to that these elements will not migrate to detrimentally affect he bond coat-oxide layer interface and accelerate corrosion of the substrate. Another example is set forth in U.S. Pat. No. 5,952,085 to Rickerby, et al. in which multiple erosion resistant coatings are applied over substrates by spattering to improve erosion resistance. These alternating layers are comprised of tungsten (W) and titanium diboride. The alternating layers are between 0.3-1 micron thick. Yet another example is set forth in U.S. Pat. No. 5,763,107 also to Rickerby, et al. This patent sets forth a variation of the (Nixe2x80x94Pt)Al bond coat in which the outer portion of the bond coat is a continuous layer of a platinum-group aluminide and the inner portion of the bond coat is substantially an aluminum-containing alloy. The continuous layer of platinum-group aluminide is covered with an adherent oxide layer of alumina. Overlying this is the ceramic thermal barrier insulating coat. While this coating may provide initial advantages, after exposure at elevated temperatures for extended periods of time, diffusion of elements should make this composition indistinguishable from conventional (Ni,Pt)Al coatings and thus subject to the problems experienced and described previously.
What is needed is a coating that can be used on turbine components that is effective in reducing the build-up of corrosive materials on all portions of the component and that can enhance the environmental protection of currently applied bond coatings and environmental coatings.
The present invention is generally applicable to components that operate in high temperature environments in which there is exposure to hot gases of combustion that can cause corrosion of the components, including oxidation such as are found in gas turbine engines. Improvements in manufacturing technology and materials are the keys to increased performance and reduced costs for many articles including those used in gas turbine engines. As an example, continuing and often interrelated improvements in processes and materials have resulted in major increases in the performance of aircraft gas turbine engines that result in even higher operating temperatures for these engines. As the operating temperatures increase, the effects of the high temperature environment are observed in regions beyond the portions extending immediately into the hot stream of gases, previously the airfoil portions of blades and vanes. Over time, these conditions ultimately can cause the deterioration of even the protective coatings applied to substrates to improve their performance. The hot gases of combustion may additionally deposit products of combustion onto surfaces of the components that may contribute to the corrosion of the component. Notable examples of such components include airfoils such as turbine blades or buckets and turbine vanes. The present invention is not limited to airfoils as other components that can benefit from the present invention include combustors, combustor liners and turbine shrouds.
Generally, the present invention is a tightly adherent coating that provides a surface that is described as anti-stick in that products of combustion and other deposits that may be present in hot gases of combustion to not deposit on it. This tightly adherent coating is applied over the component and assists the component operating at high temperatures by increasing its life expectancy. The coating can provide additional protection to the underlying component even at the elevated temperatures and extreme environments found in the portion of the turbine engine aft of the combustor. While not impervious to diffusion of oxygen, the coating does act to slow the diffusion of oxygen to the underlying coatings, thereby extending the life of these coatings by slowing the inevitable process of deterioration. Substrate articles designed to survive in this environment are comprised of superalloys having the required mechanical properties that are especially engineered and designed to be able to withstand the harsh environments and high temperatures. In order to improve the life of these alloys, coatings are applied over the superalloy substrate. The coating of the present invention can be applied directly over these tightly adherent coatings to delay their deterioration over time and increase their life expectancy.
The tightly adherent coating that overlies the substrate component is a thin, tightly adherent oxide layer formed by the oxidation of at least one metal from groups 4b or 5b, certain elements of group 4a of the Periodic Table of elements, platinum (Pt) and tungsten (W) or combinations of these metals. By proper application of these metallic oxides, the outer surface of the component exposed to the extreme environment is less resistant to accumulation of deposits resulting from exposure to the combustion gases, while diffusion of oxygen to the underlying surfaces is slowed. Because the turbine components of the present invention have reduced build-up of deposits that typically include corrosive chemicals and the diffusion of oxygen to underlying materials is reduced, the underlying component can operate for longer periods of time before requiring refurbishment or repair.
An advantage of the present invention is that the antistick coating in not only hard, durable and capable of withstanding high temperatures, but it also very light. Thus, the longer life of the components can be achieved with no weight penalties with the present invention. The very thin coating allows for its use on components have cooling channels and other openings without concern about blocking important cooling openings so that special precautions or preparation is not required for its application. The minimal thickness of the coating also has a minimal effect on the dimensioning and tolerancing of the parts to which it is applied.
Another advantage of the present invention is that it is compatible with the materials utilized in the turbine portions of gas turbine engines. These materials include the specially engineered superalloys as well as the environmental coatings and bond coatings typically applied to these superalloys to extend their life and to the ceramic thermal barrier coatings applied over these coatings to improve the thermal performance of these components.
Yet another advantage of the coating of the present invention is that it is inexpensive in comparison to the cost of coatings including MCrAlY coatings and platinum aluminide coatings. And although the coatings of the present invention are not designed to replace such coatings in the airfoil portions of components, they can extend the life of such coatings thereby reducing overall operating costs. Because the coatings of the present invention minimize the deterioration of the expensive MCrAlY coatings and platinum aluminide coatings, refurbishment of these coatings may be accomplished at reduced costs without the need for complete removal.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
Whenever possible, the same reference numbers will be used throughout the figures to refer to the same parts.